Discovery of the Most Distant Milky Way Halo RR Lyrae Stars

RR Lyrae stars are powerful tracers of Galactic structure, substructure (intact and disrupted dwarf satellite galaxies), accretion history, and dark matter content. The characteristic photometric variability pattern of RR Lyrae stars makes it relatively easy to tell them apart from other stars (sample contaminants) and they are excellent standard candles (distance indicators). Here we report the discovery of distant RR Lyrae stars, including a handful of the most distant stars known in the Milky Way halo, with distances larger than 300 kpc. The data used in this study is taken in the u*g’i’z’ bands with the Canada-France-Hawaii Telescope as part of the Next Generation Virgo Cluster Survey (NGVS). The NGVS covers 104 deg² in the Virgo direction with excellent imaging depth (10-sigma at g ~ 24.5 mag in a single exposure). We use a template light curve fitting method based on empirical SDSS Stripe 82 RR Lyrae data to identify RR Lyrae candidates from the sparse and unevenly sampled NGVS multiband data (~6-11 epochs in each filter across 4 years). When we test our detection algorithm on the NGVS light curves of 84 previously classified RR Lyraes in Pan-STARRS1 (PS1) data, we achieve a recovery rate of 98.8%, with a period match at the 0.2% level. In total, we detect 289 RR Lyrae candidates, with heliocentric distances from 20 kpc out to 320 kpc. Compared with other RR Lyrae surveys like PS1, HiTS and DES, this NGVS study has smaller sky coverage and comparable cadences, but significantly better single-epoch photometric precision, consequently making our RR Lyrae stars the most complete and robust sample at these Galactocentric distances, with the best measured pulsation parameters. These newly discovered distant stars are an important addition to the few known tracers of the outermost regions of the Milky Way halo.

YF is supported in part by the China Scholarship Council and the National Science Foundation.